An actuator arranged to drive a control shaft of a variable compression ratio mechanism is mounted to a side wall of an upper oil pan fixed to a lower portion of a cylinder block by using a plurality of fixing bolts, for example, as shown in a patent document 1. As one example of a mounting structure in which the actuator is mounted to the side wall of the main body of the engine, a patent document 2 discloses a structure in which an actuator of a variable valve actuating device is fixed on both (to extend across) two components of an upper head and a lower head.